Brain Permeability of Water

Abstract

The equilibration of labeled water (H2 15O)with the exchangeable water pool of brain was examined during a single capillary transit. The labeled water did not equilibrate freely when cerebral blood flow (CBF) exceeded 20 (ml/100 gm)/min in the rhesus monkey. At normal CBF [.. 50 (ml/100 gm)/min] only 90% of the injected bolus freely equilibrated with brain. This value progressively declined as CBF increased.(30: [319][320][321] 1974) Radi oi sotopes of water have been widely assumed to equilibrate with the exchangeable water in brain during a single capillary transit.1-3For this reason, labeled water has been used as a standard for the eval¬ uation of the blood-brain barrier transport and brain distribution of other substances4 and as a tracer for the measurement of cerebral blood flow (CBF).' We present evidence that labeled water does not freely equilibrate with the exchangeable water in brain when mean CBF ex¬ ceeds 20 (ml/100 gm)/min. MethodsThe fraction of labeled water extracted by the brain during a single capillary tran¬ sit was determined subsequent to the in¬ ternal carotid injection of 0.2 ml of whole blood labeled with H21506 in 20 adult rhesus monkeys. To facilitate the injection of the radioisotope into the internal carotid ar¬ tery, all branches of the right external ca¬ rotid artery were ligated two weeks prior to the experiments. The radioisotope was then injected into the common carotid ar¬ tery through a small (0.021 cm) catheter positioned there under fluoroscopic control from the femoral artery.The monkeys were anesthetized with phencyclidine, paralyzed with gallamine triethiodide, and passively ventilated on 100% oxygen. The end-tidal carbon dioxide pressure (Pco2), arterial blood pressure, and rectal temperature were monitored continuously. Arterial pH, Pco,, and oxy¬ gen pressure were measured before and after each injection.The time course of labeled water through the brain was detected by a 3x2 inch sodium iodide, thallium impregnated scintillation detector appropriately posi¬ tioned and collimated to insure uniform detection efficiency. The signal from the detector, after suitable electronic process¬ ing including narrow pulse height analysis, was processed in a classic small laboratory computer where corrections were made for isotope decay and room background. Tem¬ poral resolution of the data was optimized

Single‐coil arterial spin‐tagging for estimating cerebral blood flow as viewed from the capillary: Relative contributions of intra‐ and extravascular signal

“…For PS ϭ 2000, which approximates instantaneous blood-tissue equilibration, the tissue population of inverted protons is virtually linear with flow. At PS-products at the low end of the physiological range (that is, below 200 ml/100 g-min), the total inverted population in the tissue approaches an asymptote at high flows, consistent with what is known about water as a diffusionlimited indicator (9,37,38). Even though an asymptote may not be reached, for PS-products of 200 and flows above 100 ml/100 g-min, inverted tissue populations begin to be strongly nonlinear with flow.…”

“…[9] In the steady-state condition, the time derivative of Q tis equals zero. Since T R is about twice T 1sat , the steady-state condition is approximately correct, and we can write that:…”

EFFECTS OF HYPERTONIC SOLUTIONS AND OF MERCURIC CHLORIDE ON THE UPTAKE OF [14C]GLUCOSE ANALOGUES BY RABBIT BRAIN

“…Any modification of 3H20 uptake produced by the experimental condition, whether by changes in blood flow or not, could alter the BUI without actually changing 14C tracer uptake. Since under some conditions the uptake of 3H20 may be flow limited (Raichle, Eichling & Grubb, 1974), a decrease in blood flow could produce an increase in BUI without actually increasing the permeability of cerebral capillaries to the test tracer.…”

Biological Effects and Health Implications of Radiofrequency Radiation

“…Oscar and Hawkins (1977) reported quantitative measurement of microwave-induced BBB permeability increases using the Oldendorf technique with tritiated water as the internal standard. Water does not freely equilibrate in the brain and as cerebral blood ftow increases, water's diffusion is lowered (Raichle et al, 1974(Raichle et al, , 1976. Since the Oldendorf technique measures the ratio of a test substance to the internal standard as blood ftow increases and causes the brain tissue level of water to decrease, the reported ratios of BBB permeability may be overly high.…”